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If you have submodules, developer doesn't need to fork the BangleApp repository but have only its own files in his repository. Then developer request to register its app and inform when there are changes. But ok I understand it doesn't match :)
CODEOWNERS in addition to be a pointer when opening an issue is really interesting when there are contribution on an app. It should affect the PR to the codeowner automatically. At least it works like that on Gitlab.
I don't think app submodules are a good idea, they would complicate things a lot without adding much (if any) benefit: Instead of just forking the BangleApps repo, you first need to learn about submodules, then fork/create the app repo. And then fork the BangleApps repo anyway to update the submodule. (Not to mention the complications if an app maintainer seems inactive/deletes their repo, or for global changes which affect multiple apps)
CODEOWNERS looks interesting though, it could be a neat way for people to opt-in to receive notifications about certain apps, and you could ask issue submitters to look at that file for people to @mention. It seems a bit duplicate with the per-app readme/blame, but it also means people who for some reason don't want to be bothered anymore could just be removed from the CODEOWNERS file.